1. Field of the Invention
The present invention relates to power supplies. More particularly, the present invention relates to high voltage low ripple power converters.
2. Background Art
In certain applications requiring a high voltage power source, it is desirable that the power source is substantially free of ripple effects. One known method of generating regulated high DC voltage at high power level involves rectifying an AC power source (e.g., 208 VAC, 60 Hz) and then converting the rectified DC voltage into high voltage using a high frequency switching converter. In such a scheme, the low frequency ripple components, mainly the line harmonics, are attenuated by a voltage feedback loop, whereas the high frequency components, produced by the switching converter, are greatly reduced by an LC output filter. Increasing the switching frequency makes the filtering easier--the feedback loop gain can be increased which improves input ripple rejection, and the low pass filter (removing switching noise from the output) becomes more effective. On the other hand, the higher switching frequency reduces the efficiency of the power converter. The objective is to find a topology that would allow the power converter to operate at a high frequency and with a good efficiency and to produce low noise, high stability, and high voltage output.
FIG. 1 is a schematic diagram of a two-stage power converter in accordance with the prior art. The converter 10 includes a buck regulator stage 12 and a bridge stage 14. A transformer 17 is also shown. The converter operates as follows. A three-phase power source (e.g., 208 volts at 60 hertz) is applied to rectifier 16. Unregulated rectified DC power is applied to the buck regulator stage 12, which includes a single buck regulator. The buck regulator comprises a switch 20, an inductor 30, and a diode 25. The buck regulator 12 produces DC pulses that are integrated by inductor 30 and received by the bridge 14. The bridge comprises four switches 32, 34, 36, 38 which act in concert to produce an alternating current from the DC pulses.
FIG. 2A is a diagram showing the switch current in the buck regulator stage of a two-stage power converter in accordance with the prior art, as shown in FIG. 1. FIG. 2B is a diagram showing the inductor current in the buck regulator stage of a two-stage power converter in accordance with the prior art, as shown in FIG. 1. FIG. 2C is a diagram showing the transformer current in a two-stage power converter in accordance with the prior art, as shown in FIG. 1. FIGS. 2A, 2B and 2C have equivalent horizontal axes so as to better compare the wave forms. FIG. 2A shows the current through switch 20, which is operating, for example, at 100 Khz. The wave form may be described as a DC pulse train. FIG. 2B shows the current through inductor 30. The wave form may be described as an integrated DC pulse train. The bridge stage 14 requires two DC Current pulses to produce a single cycle of the output 50 KHz AC, as shown in FIG. 2C.
Since buck regulator stage 12 must produce two pulses of DC power for each cycle of the output frequency, the switch 20 must operate at twice the output AC frequency. Higher switching frequencies at switch 20 generally result in greater power loss and require more expensive switching components. Also, higher currents at inductor 30 generally require more expensive inductor components.
It is therefore desirable to minimize switching power losses, as well as to reduce component costs, in similar circuits.